1. Field of the Invention
This invention relates to a gain control circuit for the automatic record level adjustment in the sound recording circuit of a tape recorder or the like, or more in particular to a gain control circuit whose elements are adapted to be arranged in integrated circuits.
2. Description of the Prior Art
One of the conventional gain control circuits used for tape recorders or like devices is shown in FIG. 1. In this drawing, reference numeral 10 shows an input signal source for, say, an audio signal ei. Numeral 20 shows an amplifier circuit, which in this case is a differential amplifier circuit, for amplifying and introducing to the output terminal 5 the audio signal ei applied through the resistor Ra and the capacitor Ca to the input terminal (pin) 1. Reference characters Q.sub.1 and Q.sub.2 show an NPN differential transistor pair with the emitters thereof connected to each other on the one hand and to the ground terminal 6 through the resistor R.sub.5 on the other. The base of the transistor Q.sub.1 is connected to the input terminal 1 and also, through the resistors R.sub.3 and R.sub.1, to the DC power supply terminal 7 supplied with a DC voltage. The base of the transistor Q.sub.1 is further connected to the ground terminal 6 through the resistors R.sub.3 and R.sub.2. The collector of the transistor Q.sub.1 is connected through the resistor R.sub.4 to the power supply terminal 7 on the one hand and to the base of the PNP transistor Q.sub.3 on the other. The collector of the transistor Q.sub.2 is connected directly to the power supply terminal 7, while its base is connected via the resistor R.sub.6 to the terminal 4 on the one hand and to the collector of the transistor Q.sub.3 through the resistor R.sub.7 on the other. The collector of the transistor Q.sub.3 is connected to the earth terminal 6 via the resistor R.sub.9 and also to the output terminal 5. The emitter of the transistor Q.sub.3 is connected via the resistor R.sub.8 to the power terminal 7. Terminal 1 is grounded through the capacitor Ca, the resistor Ra and the signal source 10. Terminal 4 is grounded through the capacitor Cc, while the terminal 6 is grounded directly. Terminal 5 is connected to the input terminal of an amplifier circuit in the next stage (not shown). Numeral 30 shows an audio signal attenuator circuit including a controlled transistor Q.sub.4 for bypassing part of the audio signal ei to the ground terminal, and numeral 40 a rectifier circuit for detecting and rectifying part of the audio signal produced at the output terminal 5 of the amplifier circuit 20. The resulting rectified signal is used to control the controlled transistor Q.sub.4 of the audio signal attenuator circuit 30.
In the circuit configuration of FIG. 1, the part defined by a dotted line shows an integrated circuit. Assume that the audio signal ei is applied to the input terminal of the amplifier circuit 20 via the resistor Ra and the capacitor Ca. An amplified audio signal is produced at the output terminal 5 of the amplifier circuit 20. Part of the audio signal is rectified by the rectifier circuit 40, and the rectified DC signal is applied to the base of the controlled transistor Q.sub.4 of the audio signal attenuator circuit 30. As a result, the transistor Q.sub.4 is forward biased and controlled in such a manner that the collector-emitter impedance thereof decreases with the increase in the DC signal. Part of the audio signal ei from the audio signal source 10 is thus attenuated by bypassing it via the transistor Q.sub.4 thereby to control the gain of the whole circuit.
In this conventional gain control circuit in which the collector-emitter impedance of the controlled transistor Q.sub.4 is controlled so that the input signal ei is attenuated by bypassing a part of the input signal ei to the ground terminal via the transistor Q.sub.4 so as to control the gain of the whole circuit, the collector-emitter DC voltage Vce of the transistor Q.sub.4 is generally set at zero so as to provide a great change in the collector-emitter impedance of the transistor Q.sub.4 when a DC signal is applied to the base of the controlled transistor Q.sub.4. In other words, unless the DC voltage Vce between the collector and emitter of the transistor Q.sub.4 is set at zero, the collector-emitter impedance of the transistor Q.sub.4 cannot be satisfactorily changed, thus making an effective gain control impossible. For this reason, the capacitor Ca or the like is required to be inserted between the collector terminal 2 of the transistor Q.sub.4 and the input terminal 1 of the sound amplifier circuit 20 for providing DC isolation. This configuration, which presents no significant problem in the case of the circuit being comprised of discrete circuit elements, is very disadvantageous in the application of a monolithic integrated circuit thereto, because of the need for one additional terminal pin, which might involve a problem in the design of integrated circuits having a limited number of terminals.